1. Field of the Invention
The present invention relates to a method for producing a semiconductor device, such as a photodetector in which elements, including photoelectric conversion elements and thin-film transistors (hereinafter referred to as xe2x80x9cTFTsxe2x80x9d), are formed on the same substrate. More particularly, the present invention relates to a method for producing a semiconductor device represented by a photodetector used for a one- or two-dimensional image-reading device in a facsimile machine, a digital copying machine, or a scanner, or for detecting radiation (e.g., X-rays or xcex3-rays) converted to light in the photosensitive wavelength range of the photodetector by a fluorescent plate.
2. Description of the Related Art
Conventionally, reduction optical systems and reading systems using CCD sensors have been employed for facsimile machines, digital copying machines, and radiation detectors. However, recently, with the development of photoelectric conversion semiconductor materials such as amorphous silicon (hereinafter referred to as xe2x80x9caxe2x80x94Si filmxe2x80x9d), contact type linear sensors have been investigated and put into practical use, in which the sensor""s photoelectric conversion elements are formed on a large substrate so as to read information without reducing the size of the information.
In particular, when employing the axe2x80x94Si films, semiconductor layers in photoelectric conversion elements and switching TFTs can be advantageously formed at the same time because the axe2x80x94Si films can be used not only for the photoelectric conversion material but also for the semiconductor material for the switching TFTs.
FIG. 1 is a cross-sectional diagram illustrating a PIN optical sensor as an example of optical sensors employing the axe2x80x94Si films. There are shown a glass substrate 101, a lower electrode 102, a p-type semiconductor layer (hereinafter referred to as xe2x80x9cp layerxe2x80x9d) 103, an i-type semiconductor layer (hereinafter referred to as xe2x80x9ci layerxe2x80x9d) 104, an n-type semiconductor layer (hereinafter referred to as xe2x80x9cn layerxe2x80x9d) 105, and a transparent electrode 106.
FIG. 2 is a circuit diagram of the PIN optical sensor shown in FIG. 1. There are shown a PIN optical sensor 110, a power source 111, and an output circuit 112 such as a current amplifier. The C and A sides shown in FIG. 2 correspond to the sides of the transparent electrode 106 and the lower electrode 102 of FIG. 1, respectively. The voltage applied to the C side by the power source 111 is set to positive with respect to that of the A side.
The basic operation of the PIN optical sensor 110 will be briefly described with reference to FIGS. 1 and 2.
As is shown in FIG. 1, when light enters the i layer 104 from the direction shown by the arrow L in FIG. 1, the incident light is photoelectrically converted and creates electrons and holes. Due to an electric field applied to the i layer 104 by the power source 111, the electrons move toward the C side, in other words, pass through the n layer 105 to the transparent electrode 106, and the holes move towards the A side, in other words, the holes are transferred to the lower electrode 102. A photoelectric current thereby flows in the optical sensor 110.
If no light enters the optical sensor 110, no electrons and holes are generated in the i layer 104. Since the n layer 105 serves as a barrier for holes in the transparent electrode 106 and the p layer 103 functions as a barrier for electrons in the lower electrode 102, neither the electrons nor the holes can move and no photoelectric current flows in the optical sensor 110. Based on the above mechanism, the current in the circuit changes with the presence and absence of incident light. Such changes in the current are measured by the output circuit 112 shown in FIG. 2, and thus the optical sensor 110 detects incident light.
However, employing a PIN optical sensor such as shown in FIG. 1, it is difficult to achieve a photodetector having a high S/N ratio at a low cost, for the following reasons:
The first reason is that PIN optical sensors require barrier layers, i.e., p and n layers.
In the PIN optical sensor of FIG. 1, the n layer 105 must facilitate the movement of electrons to the transparent electrode 106, and simultaneously, must prevent holes from entering the i layer 104. If the n layer 105 does not exhibit one of these characteristics, the resulting photoelectric current decreases or a dark current, i.e., a current flowing when no light enters the optical sensor, appears or increases, causing a reduction in the S/N ratio.
In general, to improve the above characteristics of the n layer 105, it is necessary to optimize various conditions such as the film-forming conditions for the i layer 104 and the n layer 105 and heat-treatment conditions after film-forming.
Meanwhile, the p layer 103 must facilitate the movement of holes to the lower electrode 102, and simultaneously, must prevent electrons from entering the i layer 104. Thus, similarly to the n layer 105, various conditions for the i layer 104 and the p layer 103 must be optimized. In general, the conditions required for optimizing the n layers and those for the p layers are not the same, and thus it is very difficult simultaneously to satisfy the required conditions for the n layers and the p layers. In other words, it is difficult to produce an optical sensor having a high S/N ratio because two types of barrier layers, i.e., the p and n layers, are required to be formed in the same optical sensor.
The second reason will be explained with reference to FIG. 3. FIG. 3 is a cross-sectional diagram illustrating a switching TFT which is used in a controlling section for a photodetector. There are shown a glass substrate 101, a lower electrode 102, a gate insulating film 107, an i layer 104, an n layer 105, and upper electrodes (i.e., source and drain electrodes) 160.
The switching TFT is fabricated as follows: the lower electrode 102 functioning as a gate electrode G, the gate insulating film 107, the i layer 104, the n layer 105, and the upper electrode 160 are formed on a glass substrate 101 in the above order; the upper electrode 160 is formed into the source and drain electrodes by etching; and then, a portion of the n layer 105 is removed to form a channel 170. Since the characteristics of the switching TFT are largely affected by the conditions of the interface between the gate insulating film 107 and the i layer 104, in general, the above film-forming process is continuously carried out under a vacuum or without the workpiece being exposed to air.
If the PIN optical sensor shown in FIG. 1 is made on the same substrate on which the switching TFT is formed, production cost increases and the characteristics deteriorate. This is attributed to the differences in layer structures of the PIN optical sensor having the electrode, the p layer, the i layer, the n layer, and the electrode formed on the substrate in that order and the switching TFT having the electrode, the insulating layer, the i layer, the n layer, and the electrode formed on the substrate in that order. In other words, the PIN optical sensors and the switching TFTs cannot be formed simultaneously by the same process. Thus, the fabrication process becomes complicated such that film-forming steps and photolithographic steps are repeated for forming the required layers in the required regions, resulting in a decreased yield, higher cost, etc.
For example, when the PIN optical sensors and the switching TFTs can employ the same i layer and n layer, it is possible continuously to form the gate insulating layer and the p layer, remove portions of the p layer in the regions of the respective switching TFTs, and continuously form the i layer and the n layer, thus simplifying the fabrication process. However, the interface between the gate insulating film and the i layer, which interface is important for the switching TFT characteristics, and the interface between the p layer and the i layer in each PIN optical sensor are sometimes contaminated, resulting in deteriorated characteristics and a decreased S/N ratio.
Furthermore, if capacitors, required for obtaining the integral of the electrical charge or current generated by the PIN optical sensors, have the same structure as that of the PIN optical sensor of FIG. 1, the capacitors cannot have excellent characteristics and low leakage. This is because although a capacitor must have a barrier between two electrode layers so as to prevent both electrons and holes from moving, the semiconductor layers in the PIN optical sensor of FIG. 1 cannot sufficiently prevent movement of electrons and holes; thus an excellent capacitor with low leakage cannot be achieved.
As is mentioned above, when switching TFTs or capacitors, both of which are important elements for a photodetector, cannot be produced by the same process as that for photoelectric conversion elements, or if their characteristics do not match those of photoelectric conversion elements, the whole fabrication process inevitably becomes complicated and results in a reduction in yield.
In particular, the above fact causes a great problem in achieving a multi-functional and high-performance photodetector at low cost which sequentially detects optical signals from a plurality of optical sensors arranged one- or two-dimensionally.
Accordingly, an object of the present invention is to provide a method for producing a semiconductor device usable as a photodetector, by which method photoelectric conversion elements, each having a high S/N ratio and stable characteristics, and switching TFTs can be formed by the same process.
Another object of the present invention is to provide a method for producing a semiconductor device usable as a photodetector, by which method a photodetector having a high S/N ratio and composed of photoelectric conversion elements and switching TFTs can be produced at low cost. In this method, each of the photoelectric conversion elements is composed of a first electrode layer, an insulating layer, a photoelectric conversion semiconductor layer, a barrier layer for preventing carriers from entering the photoelectric conversion semiconductor layer, and a second electrode layer so that the photoelectric conversion elements and the switching TFTs can be produced by the same simplified process.
Another object of the present invention is to provide a method for producing a semiconductor device usable as a photodetector, by which method misaligning due to replacement of masks is avoidable because a single mask can be used for the step for forming the source and drain electrodes of switching TFTs, and for the step for removing portions of an ohmic contact layer corresponding to switching TFT channels. Thus, compact TFTs having excellent characteristics can be achieved accurate in size, and also, a semiconductor device usable as a photodetector can be produced with an improved aperture ratio.
Still another object of the present invention is to provide a method of producing a high-performance semiconductor device usable as a photodetector, according to which method elements of the device readily acquire higher performance and uniform characteristics.
Another object of the present invention is to provide a method of producing semiconductor devices at an improved yield, according to which method elements having higher performance and uniform characteristics can be produced by a decreased number of steps without complicating the steps.
An object of the present invention is to provide a method for producing a semiconductor device composed of a plurality of pixels in each of which a photoelectric conversion element having an upper electrode and a TFT having source and drain electrodes are monolithically formed on the same substrate. According to the invention, such method includes: a step for forming the source and drain electrodes of the TFT and removing a portion of at least an ohmic contact layer in a channel region of the TFT using a first mask pattern; and a step for forming the upper electrode of the photoelectric conversion element using a second mask pattern which is different from the first mask pattern.
Another object of the present invention is to provide a method for producing a semiconductor device having a plurality of pixels, in each of which at least a photoelectric conversion element having an upper electrode and a switching TFT having source and drain electrodes are monolithically formed. According to the invention, such method includes: a first step for forming a first electrode layer using a first mask pattern; a second step for forming an insulating layer, a semiconductor layer, and an n+ type semiconductor layer in that order; a third step for making a contact hole for each of the pixels using a second mask pattern; a fourth step for forming at least a second electrode layer to be formed into the source and drain electrodes of the switching TFT and removing a portion of the n+ type semiconductor layer of each of the pixels using a third mask pattern; a fifth step for forming a third electrode layer other than the source and drain electrodes using a fourth mask pattern; and a sixth step for separating the pixels and separating the photoelectric conversion element and the switching TFT in each of the pixels using a fifth mask pattern.
Still another object of the present invention is to provide a method for producing a semiconductor device having a plurality of pixels, in each of which at least a photoelectric conversion element having an upper electrode and a switching TFT having source and drain electrodes are monolithically formed. According to the invention, such method comprises: a first step for forming a first electrode layer using a first mask pattern; a second step for forming an insulating layer, a semiconductor layer, and an n+ type semiconductor layer in that order; a third step for making a contact hole for each of the pixels using a second mask pattern; a fourth step for forming a second electrode layer other than the source and drain electrodes using a fourth mask pattern; a fifth step for forming at least a third electrode layer to be formed into the source and drain electrodes of the switching TFT and removing a portion of the n+ type semiconductor layer of each of the pixels using a third mask pattern; and a sixth step separating the pixels and separating the photoelectric conversion element and the switching TFT in each of the pixels using a fifth mask pattern.
According to the present invention, photodetectors having a high S/N ratio can be achieved at low cost by a simplified method in which photodetectors are composed of a plurality of pixels each having an MIS photoelectric conversion element having a first electrode layer, an insulating layer, a photoelectric conversion semiconductor layer, a barrier layer for preventing carriers from entering the photoelectric conversion semiconductor layer, and a second electrode layer formed on an insulating substrate in that order, and a switching TFT having a first electrode layer, an insulating layer, a semiconductor layer, an ohmic contact layer for the semiconductor layer, and a second electrode layer formed on the same insulating substrate in that order.
When employing a method for producing photodetectors in which photoelectric conversion elements, each having an upper electrode, and TFTs, each having source and drain electrodes, are monolithically formed on the same substrate, different masks are conventionally used for patterning the upper electrode of each photoelectric conversion element, for patterning the second electrode layer of each TFT, and for removing portions of the ohmic contact layer each corresponding to a TFT channel of each TFT, respectively. Thus, the portions of the ohmic contact layer each corresponding to the TFT channel are removed by patterning, after patterning the second electrode. Meanwhile, according to a method of the present invention, formation of the source and the drain electrodes of each TFT and removal of the portions of at least the ohmic contact layer each corresponding to the TFT channel of each TFT are carried out by one step using one mask, and formation of the upper electrode of each photoelectric conversion element is performed by another step using another mask. Therefore, it becomes unnecessary to consider the shift caused by misaligning the mask used for forming the source and the drain electrodes and that used for removing the portions of the ohmic contact layer corresponding to the TFT channels, and in particular, margins conventionally set for the TFTs corresponding to the width of the source and the drain electrodes are not required, resulting in more compact TFTs with an improved aperture ratio.